1. Field of the Application
The present invention relates to centrifugal casting, and more particularly to a flux intended for use in centrifugal casting of bimetallic pipes.
The invention can be advantageously used in the production of compound centrifugal tubular steel castings the external layer of which are made of pearlitic steels and the internal of austenitic steels.
The invention makes it possible to manufacture bimetallic pipes by centrifugal casting from pearlitic and austenitic steels with the length-to-diameter ratio thereof being 8 to 10, while ensuring sound and uniform welding of these metals over the entire length of the casting and refining of the metals, i.e. removing non-metallic inclusions therefrom.
2. Description of the Prior Art
The production of bimetallic pipes by centrifugal casting for the need of various branches of industry is increasing on an ever greater scale in industrially developed countries the world over. The metal obtained by the method of centrifugal casting has mechanical-and-physical properties similar to those possessed by the forged metal, whereas the production process in the former case is less labor-consuming.
The problem of producing bimetallic pipes by centrifugal casting, with the length-to diameter ratio thereof being more than 8 to 10, lies in the difficulty of obtaining sound and uniform welding of the two metals throughout the casting length. One of the most important factors in the process of obtaining a high-quality external layer of steel and forming strong bonding of two metals in the compound casting is an appropriate selection of an optimum composition of flux having special physical-and-chemical properties.
It is known to select flux for each separate bimetallic compound, the flux composition depending upon the type of steel in the external layer, as well as upon the conditions of casting and pouring said steel into an iron mold.
The fluxes used in the production of compound centrifugal tubular castings should possess the following chemical-and-physical properties: the ability to refine the internal layer of metal/remove non-metallic inclusions therefrom when poured into an iron mold/; to be active or to be able to dissolve the oxide film formed on the internal surface of the external layer of metal and to prevent its oxidation and air cooling; its melting temperature, viscosity and density should be minimal, as well as its adhesion to the first or external layer of metal.
Making provision for all the above-mentioned properties of flux, requisite for the selection of its optimum composition having important bearing on the production of bimetallic pipes by centrifugal casting, it becomes feasible to substantially improve the quality of metal bonding while simultaneously increasing the length of the cast pipe.
In modern practice of producing bimetallic pipes from pearlitic and austenitic steels by the method of centrifugal casting use is made of fluxes having the following oxide-salt systems: CaF.sub.2 -Na.sub.2 O-CaO-SiO.sub.2 -B.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -chlorites (cf. USSR Inventors' Certificates No. 238,107, No. 358,075, Nos. 409,781, 309,773, U.S. Pat. No. 3,414,044, etc.,).
The compositions of fluxes mentioned above fail to have any favorable effect on the process of centrifugal casting of bimetallic pipes from austenitic and pearlitic steels, with the length-to-diameter ratio thereof being more than 8 to 10, because of the low activity of these fluxes with respect to the oxide film formed on the internal surface of the external layer of metal, and also because of high melting temperature and excessive adhesion thereof to the external metal (800 to 1200 erg /cm.sup.2).
The fluxes containing chlorites are unsuitable for use in centrifugal casting because of their high toxicity, which greatly deteriorates health conditions of work.